Known in the art is a device for determining time characteristics of a weak optical radiation ("Kvantovaya Elektronika, Moscow, 1987, v. 14, No. 6, V. F. Kamalov et al. "Lazerny subnanosekundy fluorestsentny spektrometr so schetom odinochnykh fotonov", p. 1303-1308) comprising a pulsed laser radiator with a holder for the test object, a photoelectronic multiplier, a photodiode, and a time-to-amplitude converter with its output connected to a multichannel analyzer, all arranged in the path of the laser beam. A variable-wavelength monochromator is placed before the photocathode of the photoelectronic multiplier. The start-input of the time-amplitude converter is connected to the photodiode output, and its stop-input, through a discriminator, to the anode of the photoelectronic multiplier. The laser beam is applied to the photodiode and to the test object, causing its fluorescence. The radiation from an object, of a specific wavelength to which the monochromator is turned, is converted to electric signals by the photoelectronic multiplier. The time-to-amplitude converter develops a voltage in proportion to the time period by which the instant of light quantum emission from an object is delayed relative to the instant of its irradiation (as determined by the photodiode pulse). The multichannel analyzer stores the measurement results in the channel corresponding to the time delay measured. When a sufficient number of time-to-amplitude converter outputs has been stored, the multichannel analyzer produces an amplitude-time characteristic of the radiation being investigated at a single fixed wavelength.
Known in the art is a device for obtaining spatial characteristics of weak luminescence sources (Review of Scientific Instruments, 1987, v. 58, No. 12, M. Lampton et al. "Delay line anodes for microchannel-plate-spectrometers", p. 2298-2305) comprising a photodetector with microchannel plates plates and a delay line anode system with the start-and stop-inputs of the time interval meter connected thereto through discriminators. The radiation from the test object initiates a localized charge derived from a specific point of the detector photocathode, which is applied to a point in the delay line whose coordinate corresponds to that of the charge generation point on the photocathode and consequently, to the coordinate of the luminescent point on the object. The localized charge produced in the delay line is the source of two electric pulses travelling towards its ends. The time-interval meter is started by one of these pulses and stopped by the other of them. The measured interval between the arrival times of the electric pulses coming to the ends of the delay line is a measure of the space position on the photocathode to which the photon from the test object has come. By means of storing the measurement results, it is possible to obtain information about the spatial distribution of light intensity.
So each of the above mentioned devices enables only one characteristic of the weak optical radiation to be obtained, namely: either the spatial characteristic or the time characteristic. However, in order to investigate objects whole luminescence changes its space positions with time, or is a result of superposition of several components spaced apart in time, a simultaneous measurement of both spatial and time characteristics of the radiation is necessary. So, for example, an important task of optical spectroscopy is separation of Raman scattering, fluorescence and phosphorescence spectra emitted by the molecules with different delays after pulse excitation. Besides, the glow spectra may change with time due to restructuring or energy relaxation processes. It is impossible to solve this problem by alternately measuring, first, spatial and then, time characteristics.
Also known is a device for determining weak optical radiation characteristics for an object (Review of Scientific Instruments, 1987, v. 58, No. 9, W. G. McMullan et al. "Simultaneous subnanosecond timing information and 2D spatial information from imaging photomultiplier tubes", p. 1626-1628) providing simultaneous timing and spatial information about the radiation investigated. The device comprises a photoelectronic multiplier with microchannel plates, and a two-dimensional resistive anode which is essentially a delay line. The anode has two pairs of terminals disposed on its ends at right angles to each other. The anode terminals are connected to a locating computer coupled with the personal computer.
Further, the device comprises a pulsed laser radiator for irradiating an object mounted in a holder, an avalanche photodiode to which part of the pulsed laser radiation is diverted, and a time-to-amplitude converter connected, through an amplitude pulse analyzer, to a second personal computer. The signal applied to the start-input of the time-to-amplitude converter is derived from an RC-circuit connected to the output electrode of one of the microchannel plates of the photoelectronic multiplier, while the stop-input of the time-to-amplitude converter is connected, through an amplifier and a discriminator, to the output of the avalanche photodiode generating electric pulses in synchronism with the laser radiator pulses. Since the microchannel plate pulse has a positive polarity and a very low amplitude, amplifiers, an inverter, and a discriminator are inserted between the RC-circuit and the stop-input of the time-to-amplitude converter.
The locating computer determines the two-dimensional space coordinate of the glow source or, with a spectroscopic mode of operation of the device, when the radiation to be investigated is linearly dispersed over the wavelength at the photocathode of the photoelectronic multiplier, the measurement results along one of the axes used to obtain information on the wavelength. The personal computer provides the storage and processing of the measurement result and produces the spectrum of the luminescence investigated or the space positions of the luminescence source.
The time-to-amplitude converter generates a voltage proportional to the time interval between the instant the object is irradiated and the instant the electric signal makes its appearance in the photoelectronic multiplier, which corresponds to the moment at which the object radiates a light quantum. The time-to-amplitude converter output voltage is converted to digital form by an amplitude pulse analyzer, and the conversion results are entered into the second personal computer and displayed as an amplitude-time characteristic.
One disadvantage of the device resides in its complexity due to the presence of two independent measuring channels, the first of them using signals derived from the photoelectronic multiplier anode serving to obtain the spatial characteristics of the radiation, the other one using signals from the microchannel plate electrode being designed to obtain the timing characteristics. Since these channels are not synchronized, noise pulses are apt to be detected in the space position measuring channel (i.e. coming from the anode of the photoelectronic multiplier). These noise pulses may be mistaken for a desired signal and so cause a false indication to appear.
Besides, the device fails to provide a sufficient accuracy of obtaining the time characteristics of the radiation, since a pulse from the electrode of the microchannel plate of the photoelectronic multiplier is used as the timing signal in the "time" measuring channel. This pulse has a small amplitude and is required to be subsequently amplified, so that ultimately, the timing information-bearing signal may be hard to detect against the noise background. Further, the terminal of the microchannel plate, from which the timing signal is derived, is at a potential relative to the case of the device (i.e. the anode of the photoelectronic multiplier), thus complicating the maintenance of the device.